As the guessing game continues about Mitt Romney's choice of a running mate, Ohio Sen. Rob Portman invariably comes up as a top contender. And with a wealth of experience in Washington and beyond, Portman would be considered a safe pick to run for vice president on the Republican ticket.

In an elevator at the U.S. Capitol, Portman, 56, says he picked up his Mexican-accented Spanish working with cowhands along the Southern U.S. border three decades ago. The multimillionaire then drapes his arm over the shoulders of a Democratic colleague standing next to him and asks how he's doing.

Portman's public display of affection for West Virginia Democratic Sen. Joe Manchin is not unusual, not for Portman, anyway. He's one of the few Republicans in Congress who seems perfectly at ease palling around with members of the other party.

'I've Got A Big Job Here'

Riding the underground train over to his office, Portman insists he's truly happy just being a U.S. senator.

"I've got a big job here and this is, you know, where I can help. I have 12 years in the House, and, now, some time here in the Senate, of reaching across the aisle, working with the other side," says Portman.

If you talk to Democrats, they say they genuinely like Portman.

"I find him to be a serious legislator who's focused on results," says Sen. Ben Cardin, D-Md., who adds that Portman shows a willingness to listen to all sides.

"I think he'd be a great choice" for Romney's running mate, says New Hampshire Democratic Sen. Jeanne Shaheen. She notes that Portman was a willing recruit when she recently asked him to co-sponsor a bill promoting greater energy efficiency.

"I knew he had a connection to New Hampshire. He went to Dartmouth, and so I called to see if he would be interested. And he is," says Shaheen.

Portman could be a vice president who would be highly effective working with Congress, says Sen. John Cornyn, R-Texas.

"I would think he would have a lot to offer from his service as a congressman and as a senator and his administration experience," says Cornyn.

Serving Both Presidents Bush

Portman served in the administrations of both presidents Bush.

He worked as an associate counsel in the George H.W. Bush administration. Later, the self-described fiscal conservative served as trade representative and then budget director in the second term of George W. Bush's presidency.

What that means, says Democratic Sen. Sherrod Brown — the other senator from Ohio — is that Portman will have a lot of explaining to do if he is chosen as Romney's running mate.

"Rob's going to have the problem of defending Bush policies on trade and budget," says Brown. "Two of the biggest negatives that, next to the war in Iraq, that the Bush administration bequeathed to us."

When asked about his time serving as budget director, Portman expresses no regrets.

"I'm very proud of our record there, which was, I was there for about 14 months and we not only submitted a balanced budget, it was a balanced budget over five years, which is harder to do than over 10 years, which is what the president just submitted," says Portman. "The difference is, mine was balanced; his had an $11 trillion debt increase."

But it's also true that the budget Portman drew up resulted in a $400 billion deficit the next year.

'Sort Of A Low-Key Style'

As a soft-spoken former corporate lawyer, trade specialist and numbers cruncher, Portman may not bring a lot of sizzle to a Romney ticket.

But retired Ohio State political scientist Herb Asher says that may not matter.

"He's not a firebrand, but when I've heard him speak, I've heard him to be a very, actually, a very effective, very compelling speaker, but obviously with a, you know, sort of a low-key style," says Asher.

Portman's already been out stumping for Romney. He was alongside the candidate last month in Newark, Ohio, where he asked a crowd: "Do you agree with me that we cannot afford four more years of Barack Obama?"

Experts say putting Portman on the ticket could help swing the key state of Ohio in Romney's favor next fall. But Portman plays down being Romney's running mate.

"He's got a lot of great choices and I don't expect to be asked," says Portman.

They don't call Portman low-key for nothing.

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ROBERT SIEGEL, HOST:

While Romney defends his record and his tax returns, he also has an important choice to make, a running mate. In the veep guessing game, one name invariably comes up as a top contender, Ohio Republican Senator Rob Portman. With a wealth of experience in Washington and beyond, Portman is considered a safe pick. But does he have sizzle? NPR's David Welna profiles this mild-mannered senator from Cincinnati.

DAVID WELNA, BYLINE: In an elevator at the U.S. Capitol, one of the occupants is talking about where he learned to speak Spanish.

SENATOR ROB PORTMAN: (Spanish spoken)

WELNA: Senator Rob Portman says he picked up his Mexican-accented Spanish working with cowhands along the Southern U.S. border three decades ago. The 56-year-old multimillionaire then drapes his arm over the shoulders of a Democratic colleague standing next to him.

SENATOR JOE MANCHIN: Hey.

PORTMAN: Hey, Joe.

MANCHIN: Hey, Rob.

PORTMAN: You going Thursday? You're going out?

MANCHIN: I think...

WELNA: Portman's public display of affection for West Virginia Democrat Joe Manchin is not unusual, not for Portman, anyway. He's one of the few Republicans in Congress who seems perfectly at ease palling around with members of the other party. Riding the underground train over to his office, Portman insists he's truly happy just being a U.S. senator.

PORTMAN: And I got a big job here, and this is, you know, where I can help. I have 12 years in the House, and now, some time here in the Senate of reaching across the aisle, working with the other side. You just saw me with one of my Democrat friends.

WELNA: And if you talk to Democrats, they say they genuinely like Rob Portman. Maryland Senator Ben Cardin says he always shows a willingness to listen to all sides.

SENATOR BEN CARDIN: So I find him to be a serious legislator who's focused on results.

SENATOR JEANNE SHAHEEN: I think he'd be a great choice.

WELNA: That's New Hampshire Democratic Senator Jeanne Shaheen on Portman as a running mate for Mitt Romney. She says Portman was a willing recruit when she recently asked him to cosponsor a bill promoting greater energy efficiency.

SHAHEEN: I knew he had a connection to New Hampshire. He went to Dartmouth. And so I called to see if he would be interested. And he is.

WELNA: And Portman could be a vice president who'd be highly effective working with Congress, says Senator John Cornyn, a Republican from Texas.

SENATOR JOHN CORNYN: I would think he would have a lot to offer from his service as a congressman and as a senator and his administration experience.

WELNA: Portman served in the administrations of both Presidents Bush. The self-described fiscal conservative was first Trade representative and then Budget director in the second term of George W. Bush's presidency.

What that means, says Democrat Sherrod Brown, who's the other senator from Ohio, is that Portman will have a lot of explaining to do if he is chosen as Romney's running mate.

SENATOR SHERROD BROWN: Rob's going to have the problem of defending Bush policies on trade and budget, two of the biggest - I think two of the biggest negatives that - next to the war in Iraq - that the Bush administration bequeathed to us.

WELNA: When asked about his time serving as Budget director, Portman expresses no regrets.

PORTMAN: I'm very proud of our record there, which was - I was there for about 14 months. And we not only submitted a balanced budget, it was a balanced budget over five years, which is harder to do than over 10 years, which is what the president just submitted. The difference is mine was balanced. His had an $11 trillion debt increase.

WELNA: But it's also true that the budget Portman drew up resulted in a $400 billion deficit the next year. As a soft-spoken former corporate lawyer, trade specialist and numbers cruncher, Portman may not bring a lot of sizzle to a Romney ticket. But retired Ohio State political scientist Herb Asher says that may not matter.

HERB ASHER: He's not a firebrand. But when I've heard him speak, I've heard him to be a very - actually, a very effective, very compelling speaker, but obviously, with a, you know, sort of a low-key style.

WELNA: Portman's already been out stumping for Romney. He was alongside the candidate last month in Newark, Ohio.

PORTMAN: As you know, in two of the last three presidential elections, Ohio decided who would be the next president of the United States. You agree with me we cannot afford four more years of Barack Obama?

UNIDENTIFIED GROUP: Yes.

(APPLAUSE)

PORTMAN: Well, folks, then let's decide this one for Mitt Romney.

(APPLAUSE)

WELNA: Experts say putting Portman on the ticket could help swing the key state of Ohio in Romney's favor next fall. But Portman plays down being Romney's running mate.

PORTMAN: He's got a lot of great choices, and I don't expect to be asked.